Spontaneous spin current near the interface between ferromagnets and unconventional superconductors

TL;DR

This paper theoretically investigates the proximity effects and spontaneous spin currents at the interface between ferromagnets and unconventional superconductors with broken time-reversal symmetry, revealing conditions for spin current flow.

Contribution

It introduces a numerical analysis of the Bogoliubov de Gennes equations for ferromagnet-unconventional superconductor interfaces, highlighting the emergence of spontaneous spin currents in specific pairing symmetries.

Findings

Spontaneous spin current flows at the interface with chiral p-wave superconductors.

No spin current arises at the d-wave superconductor interface with certain surface states.

Surface state symmetry determines the presence or absence of spontaneous current.

Abstract

Proximity effects between ferromagnets (F) and superconductors (S) with broken time-reversal symmetry (T) are studied theoretically. For the S side we consider a chiral (p_x \pm ip_y)-wave, and a d_{x^2-y^2}-wave superconductor, the latter of which can form T-breaking surface state, i.e., (d_{x^2-y^2} \pm is)-state. The Bogoliubov de Gennes equation which describes the spatial variations of the superconducting order parameter and the magnetization is derived and solved numerically. It is found that a spontaneous spin current flows along the interface between the (p_x \pm ip_y)-wave superconductor and the ferromagnet. On the contrary, in the case of a [110] interface of the d_{x^2-y^2}-wave SC, the surface state has a (d \pm p_x \pm p_y)-wave (or (d_{x^2-y^2} \pm is)-wave) symmetry, and thus no (only charge) spontaneous current arises.